The present invention refers to a work piece carrier, having a central aperture for receiving the work piece as well as two reference systems, each incorporating positioning elements, to releaseably attach the work piece carrier to a chuck member in two different, well defined positions. The present invention also refers to a clamping apparatus, comprising a chuck member and a work piece carrier that can be attached thereto in two different positions.
Well known in the prior art are clamping apparatuses to attach a work piece to a chuck member of a machine tool with high precision by means of a work piece carrier. Thereby, the work piece carrier shall not only be attached to the chuck member with high absolute precision, but particularly also a high repetitional accuracy. In order to enable the work piece carrier to be attached to a chuck member in an exactly defined position, the work piece carrier usually is provided with a reference system that is matched to a reference system provided in the chuck member; in other words, the reference system of the chuck member is correspondingly designed. Once the work piece carrier is attached to the chuck member, the work piece carrier shall take an exact, well defined and well known position at least in the X- and Y-directions. Preferably, the work piece carrier should also be aligned in the Z-direction as well as regarding its angular orientation around the Z-axis once it is attached to the chuck member.
The European Patent Publication EP-A-0,255,042 discloses a clamping apparatus for a tool of a machining apparatus, comprising a chuck member and a tool holder. The chuck member is provided with two pairs of centering ribs protruding from its bottom surface and having rest surfaces for aligning the tool holder in the X- and Y-direction. Moreover, four pins are provided, protruding from the surface of the chuck member, which are responsible for aligning the tool holder in the Z-direction. The tool holder is provided with a flat surface adapted to rest against the front faces of the above mentioned pins. Further, the tool holder comprises two pairs of grooves, aligned with the aforementioned centering ribs, and having elastic lips adapted to engage the rest surfaces of the aligning ribs. Finally, the tool holder is provided with a central bore in which a pull-down bolt is received, by means of which the pull-down force required for a positionally accurate centering of the tool holder can be transformed to the tool holder. Thereby, the chuck member comprises a centrally located ball clamping mechanism cooperating with the above mentioned pull-down bolt.
A disadvantage of such a known apparatus is, amongst else, that a work piece fixed to the work piece carrier can be machined only on one side. If, for example, a work piece has to be machined on two sides, it has to be released from the work piece carrier once the one side has been machined, and fixed to the work piece carrier again in a different position in which the other side can be machined. It is understood that such a proceeding, on the one hand, is time consuming and, on the other side, brings along inaccuracies as far as the exact position of the work piece is concerned.
The German Patent Publication DE-C-861,948 discloses a chuck member to be attached to the spindle of a machine tool which can be turned over from one position into a second position. The chuck member comprises a clamping frame provided with clamping jaws. The clamping frame is swivel-mounted by means of two bolts to be turned over by 180°, such that work pieces clamped in the clamping frame can be turned over by 180°. For positioning the clamping frame with regard to the spindle head, the latter one is provided with an annular rib, engaging annular grooves provided on both sides of the clamping frame. For fixing the clamping frame to the spindle head, the clamping frame is provided with holding elements which are engaged in each case by a screw bolt. The screw bolts are connected to a carrier member which is movable between a turn-over position and an operating position by means of a rotating and, thereby, axially moving sleeve member. Moreover, the holding elements each comprise a protruding portion, shaped correspondingly to a matching recess provided in the spindle head to secure the clamping frame against rotation once it is in its operating position.
Even if the annular rib engaging the assigned annular groove allows for some kind of positioning of the clamping frame with regard to the spindle head, no means are provided for exactly and accurately aligning the clamping frame with regard to the spindle head both in the X- and Y-directions as well as in the Z-direction. By providing independent centering elements, particularly the protruding portions engaging the recesses, as well as the annular rib engaging one of the annular grooves, the system is over determined. Moreover, the chuck member cannot be released from the spindle and, if required, attached to the spindle of another machine tool. Since the clamping frame is held only by means of two screw bolts, no high forces can be transmitted, particularly not during machining a work piece received in the clamping frame. A further disadvantage may be seen in the fact that a work piece received in the clamping frame is limited in size; if it is too large, particularly too long, the clamping frame cannot be turned over by 180° anymore without removing the work piece.
The Document EP-A-0,900,618 discloses a carrier plate having two flattened portions. The rearward portion comprises two holders located opposite to each other by means of which the carrier plate can be clamped to the chuck member in two different positions. The holders are designed according to the previously discussed document EP-A-0,255,042 and are attached to the chuck member by means of a centrally located clamping bolt.
U.S. Pat. No. 3,023,015 discloses a reversible bit attachment for a drill, comprising a tubular holder having an open forward end and a rear end. Drill chuck engaging means are provided on the rear end of the holder. A dual bit member comprises a dual socket having first and second ends which are optionally engageable in the open end of the holder. An annular stop shoulder intermediate the ends of the socket is engageable with the forward end of the holder. Releasable detent means are provided, having cooperating components on opposite sides of the holder and on opposite sides of the dual socket. A sleeve is slidably circumposed on the holder and operatively engageable with components of the detent means.
Finally, the document EP-A-0,970,769 discloses a holder for drill tools, having a central through bore constituting on each side an axial receiving hole for receiving a drill bit. The holder is provided with a radial blind hole, intersecting the two receiving holes and serving for constituting or receiving a stop member for securing the drill bit against rotation.